Patent GB-1384229-A (GB)
Title: Opthalmic field of view tester
IPC: G01R13/22
Filed: 1970-11-09
Abstract: 1384229 Cathode-ray tube displays G B B CHAPLIN S MARLOWE and J H EDWARDS 27 Oct 1971 [9 Nov 1970] 53298/70 Heading H4T An opthalmic field-of-view tester comprises means for displaying on a cathode-ray tube, during non-overlapping intervals of time, individual ones of a sequence of test points at random co-ordinate positions, means for varying the brightness of each test point during its display interval and means responsive to a patient viewing the tube screen for storing the co-ordinates of each test point and the brightness level at which it was seen. An additional feature comprises displaying, before the display of each test point, a central "fixation-point" which is displayed at a variable time interval before the presentation of each test point. As shown, the eight bit output of a pseudo random number generator 5 is compared in comparator 6 with the number in an address register 7 which receives the output of three parallel 256 bit shift registers 8 continually cycled via a control 9 from a master control 20. On equivalence of the two inputs, comparator 6 supplies an output via a variable delay 10 to the fixation-point generator 4 to display the fixation point on a conventional television receiver 1 modified to operate with 256 lines. The output from delay 10 is supplied to a second variable delay 11 which controls a three bit brightness counter 13 driven by a low frequency oscillator 13 and providing a digital output on three lines representing eight increasing levels of brightness, the output being supplied to an AND gate 15. The latter is then "opened" to pass the successive brightness levels via selector 16, flicker unit 17, inhibit gate 18 and D/A converter to produce the test points by intensity modulation of the raster. The positions at which the test points are displayed are determined by comparing the analogue value (derived via D/A converter 23), of the first four bits of the pseudo-random number generator 5, with the output of a ramp generator 25 synchronized by the X synch. of set 1 (derived from master synch, unit 2), in comparator 24 which provides one input to an AND gate 26 receiving as second input the output of digital comparator 27 which compares the remaining four bits of generator 5 with a divided by sixteen line count representing the particular line being scanned the output of AND gate 26 triggering a mono-stable 28 which controls the width of the test point by the interval for which it holds AND gate 15 open.  In operation, the central fixation-point is first displayed and after an unspecified time a test point is displayed which increases in brightness progressively. When the patient sees  the test point he presses a button 21 which operates via OR gate. 22 to extinguish the fixation-point and to stop brightness counter 13 so that the test point is extinguished. Should the patient not see the test point by the time it has reached its seventh brightness level it is assumed that he will not see it (at all) and a detector 34 responds and supplies air output to OR gate 22 which also initiates the above sequence. When the patient indicates, by pressing button 21, that he has seen a test point the brightness reached is stored in the shift registers 8 using one bit per register of the three registers as a parallel store so that the coordinates of the test point and the brightness level at which it was seen are stored. The operation continues in this manner until a choesn test sequence comprising displaying a succession of fixation-points each followed by a test point in random positions has been completed. The system may then be operated in a play-back mode to display a plot of the patient's area of vision. The information stored in (memory) shift regisrers 8 may be recorded on magnetic tape or photographic film for storage and subsequent use.